1. Field of the Invention
This invention relates to a dental composite restorative material and, more particularly, to a dental composite restorative material that has good transparency and opalescent effect and that is able to give cosmetics when it is used for the restoration of the incisal edge of tooth.
2. Description of the Related Art
Since a dental composite restorative material is able to give color tone equivalent to natural tooth color or is easy to use for treatment, it has rapidly spread in recent years and is now applied mainly for treatment of front teeth. Further, a dental composite restorative material having excellent mechanical strength applicable for the restoration of posterior teeth subject to high occlusion pressure is being developed.
In the case of the restoration of teeth by a dental composite restorative material, not only the recovery of occlusion, but also cosmetics, that is, whether a tooth restored by a dental composite restorative material looks as a natural tooth is important. It is, therefore, required for a dental composite restorative material to reproduce precisely the color tone of the treated point of a tooth.
A natural tooth comprises dentine and enamel, and the color tone of natural teeth is originated mainly from dentine and is observed through transparent enamel. Enamel increases from the tooth neck toward the incisal edge. Particularly, a large part of the incisal edge is composed of enamel and has high transparency compared with the rest of the tooth and is known for the generation of characteristic milky-white light. That is to say, the incisal edge generates a characteristic light scattering phenomenon (opalescent effect) identical to the mineral opal, and light of short wavelength is reflected by scattering; on the other hand, light of long wavelength transmits. As a result, under a bright environmental light, bluish milky-white color is usually observed, which changes to an orange-tinged color tone depending on the viewing angle. For cosmetic restoration, delicate tone of such an incisal edge should be reproduced. Not only high transparency, but also an opalescent effect is required for a dental composite restorative material applied to such an incisal edge.
In general, a dental composite restorative material comprises a polymerizable monomer, a filler and a polymerization catalyst. The above described opalescent effect can be obtained depending on the selection of filler used. That is to say, it has been reported that an opalescent effect can be obtained by the addition of a spherical inorganic filler having a particle diameter of 0.2˜0.6 μm (See Patent Publication (Toku-hyo) 2003-511400, claim) or addition of spherical inorganic filler having an average particle diameter of 0.18˜0.28 μm (See Patent Publication (Toky-hyo) 2007-532518 at para. [0032]). Hereinafter, an inorganic filler having such an opalescent effect is referred to as “opalescence-giving filler.” It is said that the opalescent effect can be clearly developed when the shape of the opalescence-giving filler is spherical as described above and its particle diameter distribution is sharper. Further, an inorganic filler having a small difference in the refractive index between the inorganic filler and the cured body of the polymerizable monomer is preferable, because it is possible to make the cured body of the restorative composite material highly transparent and to develop the opalescent effect easily. In this sense, a silica-based particle is suitable.
Patent Publication (Toky-hyo) 2007-532518 at para. [0040] describes that an organic-inorganic composite filler may be added as an optional additive other than the above-described opalescence-giving filler. However, this publication describes for an inorganic filler dispersed in the organic resin matrix in the organic-inorganic composite filler, only that pulverized glass, a silica particle, a radiopaque filler, etc. are used for the purpose of increasing mechanical strength or giving radiopacity, but never describes the dispersion of the opalescence-giving filler.